Touchscreens, which as used herein include touch sensors, are transparent or opaque input devices for computer and other electronic systems. Transparent touchscreens are generally placed over display devices such as cathode-ray-tube monitors and liquid crystal displays. In this manner, touch display systems are created which are increasingly used for commercial applications including restaurant order entry systems, industrial process control applications, interactive museum exhibits, public information kiosks, pagers, cellular phones, and personal digital assistants.
Presently, the dominant touch technologies are resistive, capacitive, infrared and acoustic. Such touchscreens have delivered high standards of performance at cost-competitive prices. Acoustic touchscreens, also known as “ultrasonic” touchscreens, have been able to compete effectively with the other touch technologies. This is due in large part to the ability of acoustic touchscreens to handle demanding applications with high-transparency, high-resolution touch performance, while providing a durable touch surface. As acoustic touchscreen technology continues to develop, a durable touch surface is expected to remain a key selling feature and product requirement.
Touchscreens based on the use of acoustic waves are touch-sensitive substrates in which the acoustic wave is propagated in a substrate, and a touch at a position on the substrate results in absorption of at least a portion of the wave propagated in the substrate. The touch position is determined by using electronic circuitry to locate the absorption position in an XY coordinate system. A common type of acoustic touchscreen employs Rayleigh type acoustic waves, which as used herein includes quasi-Rayleigh waves. Illustrative disclosures relevant to Rayleigh wave touchscreens include U.S. Pat. Nos. 4,642,423; 4,645,870; 4,700,176; 4,746,914; 4,791,416; Re 33,151; 4,825,212; 4,859,996; 4,880,665; 4,644,100; 5,739,479; 5,708,461; 5,854,450; 5,986,224; 6,091,406; 6,225,985; and 6,236,691. Acoustic touchscreens employing other types of acoustic waves such as Lamb or shear waves, or combinations of different types of acoustic waves (including combinations involving Rayleigh waves) are also known, illustrative disclosures including U.S. Pat. Nos. 5,591,945; 5,854,450; 5,072,427; 5,162,618; 5,177,327; 5,329,070; 5,573,077; 6,087,599; 5,260,521; and 5,856,820. All of the above-cited patents are incorporated herein by reference.
Acoustic touchscreens, including the Elo TouchSystems, Inc. IntelliTouch® products which sense touch via absorption of Rayleigh waves have proven to be commercially successful. The success of products using Rayleigh waves is due in large part to the following two properties of Rayleigh waves. First, Rayleigh waves have a higher sensitivity to touch than other acoustic waves. Second, Rayleigh waves are surface waves that can propagate on the surface any simple homogenous glass substrate.
Rayleigh waves, however, also have drawbacks. For example, in order for Rayleigh waves to propagate in a touchscreen substrate, the substrate generally must be three to four times thicker than the wavelength of the wave imparted into the substrate. Therefore, Rayleigh waves require relatively thick heavy substrates, which are not conducive for a variety of applications including portable electronic devices having touchscreens.
In addition, because Rayleigh waves are confined at or near the surface of the substrate, Rayleigh wave touchscreens have increased sensitivity to liquid contaminants such as oil and water and other materials abutting the touchscreen surface, such as sealants. Contaminants or other abutting materials absorb energy from the propagating waves and may cause acoustic shadows or blind spots extending along the axes of the substrate that intersect the contaminant. As a result, the touchscreen substrate cannot detect a touch along blind axes. Thus, there is a need for particular care in exposing Rayleigh wave touchscreens to water and designing the seals between the exposed touch surface and enclosed areas of the touchscreen substrate. For applications subject to high levels of liquid contaminants, use of Rayleigh waves becomes problematic.
The ability to optimize the performance of a Rayleigh wave touchscreen is limited because touch sensitivity and minimum touch panel thickness are not independent choices. In order to support Rayleigh waves in a touchscreen of reduced thickness, its other dimensions remaining the same, the wavelength or frequency must be reduced to preserve single surface confinement of the acoustic wave. It is characteristic of Rayleigh waves that their confinement depth is related to wavelength, with confinement depth decreasing as wavelength is reduced. As a result, the wave is confined to a shallower region bounded by the surface, and the proportion of wave energy absorbed by a given absorbing medium is increased. Experimentally, this is found to vary by approximately the inverse square of the wavelength. As previously discussed, touchscreens using Rayleigh waves can be considered unduly sensitive for some applications, even for relatively thick panels; hence, the effect of reducing touch panel thickness results in touchscreens which are even more sensitive to surface contaminants and other abutting materials. Conversely, reducing the sensitivity by increasing the quasi-Rayleigh wavelength results in increased panel thickness and weight. Finally, commercial economics of touchscreen mass production often favor the use of the same electronics at the same operating frequencies for a broad line of touchscreens. Thus, there is a need for a Rayleigh wave touchscreen in which the sensitivity, minimum thickness, and weight can be varied without changing the wavelength of the waves.
Shear wave mode touchscreens are also well known in the art. These systems operate by exciting zeroth order horizontally polarized shear waves (ZOHPS), which are non-dispersive, in a substrate. Such touchscreens are described and illustrated in U.S. Pat. Nos. 5,177,327 and 5,329,070 both to Knowles and were commercially sold by Exzec Inc. and Carroll Touch Inc. under the name SureTouch™.
Some of the disadvantages of Rayleigh wave touchscreens can be avoided if horizontally polarized shear waves, which may also be referred to herein as “shear waves,” are used in the touchscreen. For example, shear waves can be supported in arbitrarily thin substrates, and in fact, the substrate should be maintained at a thickness less than about two wavelengths in order to suppress higher order wave modes and overtones. Thus, shear wave touchscreens are well suited for applications in which the weight of the touchscreen must be minimized.
However, very thin substrates, such as 1 mm thick soda-lime glass originally used in shear wave touchscreens, are not durable. In order to increase the thickness and hence the durability of the 1 mm glass substrate, the substrate in which the shear wave propagated was laminated to a back plate using a non shear-wave-coupling adhesive. A suitable adhesive used to bond the substrate to the back plate is a silicone rubber adhesive which remains liquid-like even after curing.
U.S. Pat. No. 5,856,820 to Weigers et al. discloses a process used for laminating shear wave supporting SureTouch™ substrates. The laminated SureTouch™ substrate, however, cannot withstand the chemically etched and inlayed reflective array process also disclosed in U.S. Pat. No. 5,648,643. For example, silicone rubber will not survive the high sintering temperature needed to cure the silver frit inlay material disclosed in the patent. Therefore, the disclosed lamination process is a downstream manufacturing process, which occurs after the fabrication of reflective arrays in the 1 mm thick acoustic substrate. The expense of a downstream lamination process adds significant cost to the manufacturing process. There remains a need to provide a durable shear wave supporting touchscreen substrate which is water-resistant, but which can be manufactured without the expense of a downstream lamination process.
U.S. Pat. No. 5,591,945 to Kent discloses a shear wave touchscreen constructed from a 2.3 mm thick glass substrate. This touchscreen uses higher-order-horizontally-polarized-shear wave (HOHPS) to sense a touch. As set forth in this patent, Rayleigh waves propagate along the arrays and are mode converted to an n=4 HOHPS waves by non-45° reflectors.
For many applications, a 2.3 mm thick glass substrate is structurally strong enough to eliminate the need for laminated substrates; however, such HOHPS supporting touchscreens also have disadvantages. For example, a 2.3 mm thick glass HOHPS supporting substrate also supports many other additional acoustic modes including other orders of HOHPS waves, the ZOHPS waves, Rayleigh waves, and a large number of Lamb waves. Such additional acoustic modes can lead to undesired parasitic acoustic signals. HOHPS wave based acoustic touchscreens have yet to be demonstrated in the marketplace. Accordingly, there remains an unmet need to provide a low-cost yet durable touchscreen substrate, which supports desired water-resistant shear waves, while supporting a minimum of other extraneous acoustic modes.
Unlike Rayleigh wave touch screens, shear wave touchscreens can properly reconstruct touch positions even in the presence of high levels of surface contamination. Operation continues even with complete submersion of the touch surface under water. The physical mechanism for the relative insensitivity to water and other contaminants by horizontally polarized shear waves is set forth below. In contrast to Rayleigh waves, shear waves induce only horizontal motion in the substrate and have no vertical motion at the surface of the touchscreen substrate. As a result, Rayleigh waves and shear waves have contrasting touch sensing properties. The vertical motion of the substrate surface associated with Rayleigh waves results in absorption via radiation of pressure waves into the contacting media. Absorption by a finger touch and a water drop of the same contact area are similar. Shear waves, however, do not radiate pressure waves into the contacting media, but rather are absorbed mainly by viscous damping. As water is much less viscous than finger flesh, a shear wave will respond much less to a water drop than a finger touch. For acoustic touchscreen applications subject to significant contamination from water or other liquids, horizontally polarized shear waves provide a major advantage over the use of Rayleigh waves.
Although shear waves have reduced sensitivity to water and other contaminants, shear waves also tend to be much less sensitive to touch than Rayleigh waves. The percentage of intercepted energy absorbed by a given touch is about five times greater for a Rayleigh wave than it is for a comparable ZOHPS shear wave for practical touchscreen substrate thicknesses. In order to compensate for this difference in fundamental acoustic sensitivity, special controllers are needed for a touchscreen using ZOHPS waves. These controllers are more complex than those needed for Rayleigh wave touchscreens and, therefore, add cost to the controller design. Thus, it would be desirable to have the benefits of a shear wave touchscreen without a corresponding reduction in the raw touch signal strength so that less complicated and less expensive controllers may be used.
Love wave touchscreens are also known in the art, but have yet to be commercially developed. In general, a Love wave is a horizontally polarized shear wave having wave energy at one surface of the substrate and substantially less energy on the opposing surface. Like a Rayleigh wave, a Love wave is bound to the touch surface and decays in an exponential fashion with depth. Unlike a Rayleigh wave, Love waves do not exist in a homogenous medium. Mathematically, the simplest substrate supporting a Love wave is an upper layer of finite thickness bonded to a lower semi-infinite medium exhibiting a faster bulk shear wave velocity. Practically speaking, the lower medium may be a layer of finite thickness provided it is thick enough to contain a large number of exponential decay lengths of the wave amplitude. More complex structures with multiple layers may also support Love waves.
The use of Love waves as a means to allow operation in the presence of low-viscosity fluids is well known in the field of acoustic chemical sensors. See, for example, Gizeli et al., “Novel Love-plate acoustic sensor utilizing polymer overlays”, Trans. UFFC September 1992, p. 657; Jakoby and Vellekoop, “Analysis and Optimization of Love Wave Sensors”, Trans. UFFC September 1998, p. 1293; and Jakoby and Vellekoop, “Analysis of Viscous Losses in the Chemical Interface Layer of Love Wave Sensors”, Trans. UFFC May 2000, p. 696. While these disclosures illustrate basic principles, they are rather far removed in a practical engineering sense from touchscreen technology. The active areas of chemical sensors are typically very small, e.g. fractions of a square inch, and transparency is irrelevant. Love wave substrates for chemical sensors are typically piezoelectric in order to enable use of interdigital transducers. In contrast, touchscreens are generally many tens to hundreds of square inches in area. For economic reasons, substrates for commercial acoustic touchscreens are not made of piezoelectric materials, and transparency is essential for many applications. Thus, chemical sensor prior art gives few clues how to cost-effectively design and manufacture Love wave substrates for acoustic touchscreens.
Love wave touchscreens have been disclosed implicitly and explicitly in the prior art. For example U.S. Pat. No. 5,329,070 to Knowles can be interpreted as disclosing a Love wave substrate in which a slower shear velocity medium is bonded on a back plate of faster shear velocity. U.S. Pat. No. 5,591,945 to Kent discloses the use of Love wave supporting substrates comprising a 2 mm or 3 mm thick borosilicate glass bonded to 3 mm thick soda-lime glass. In addition, U.S. Pat. No. 5,854,450 to Kent discloses that that a Love wave substrate can contain laminations of more than two layers. U.S. Pat. No. 5,854,450 discloses possible Love wave substrate constructions including a 100 micron lead-based frit layer on 2 mm thick glass, enamel on aluminum, and glaze on ceramic.
Substrates for touchscreens placed in front of display devices must be transparent. In a touch display system, it is desirable to provide the touch input function with minimal impact on the quality of the displayed image as seen by the user. However, for some applications, touch input devices need not be transparent. For example, consider track pads, which like a mouse, allow the user to control a cursor. For certain market applications, there may be an opportunity for improved acoustic substrate designs even if they are opaque.
As discussed in U.S. Pat. No. 5,854,450, acoustic sensors can be designed in a variety of shapes for a variety of applications and may have application beyond user control of a computer-based system. For example, the cylindrical sensor shown in FIG. 19 of U.S. Pat. No. 5,854,450 could be a partial metal shell of a robot arm. Thus, large exposed areas could be rendered touch sensitive and used for collision detection. Sensor information could be used in a robot system to decide when to abort or modify robot arm movements. If the application demands rejection of false collision information due to water contamination, then it is advantageous to use a shear wave mode. This is another touch sensor application that may benefit from improved touch substrates.
Accordingly, it is believed that there is a need for improved acoustic touchscreen substrates which support Rayleigh and Love waves, are durable and have increased sensitivity to touch, but which are still relatively insensitive to water and other contaminants. It is further believed that there is a need for such touchscreens substrates, which can be easily manufactured at a low cost.